Circulator

ABSTRACT

A circulator suitable for use in an external field, particularly of a magnetic resonance apparatus, has a planar ferrite structure. Instead of a permanent magnet, a field conducting device is provided that is designed to strengthen or weaken an external field, and/or at least one electromagnet is provided for generating an operating field. The operating field proceeds perpendicularly through the ferrite structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a circulator suitable for use in an externalmagnetic field, in particular of a magnetic resonance apparatus, of thetype having a flat ferrite structure.

2. Description of the Prior Art

To excite nuclei in magnetic resonance and to acquire the responsesignals, magnetic resonance apparatuses employ radio-frequency antennas,in particular whole-body antennas that are arranged inside the magnetand must be activated with correspondingly high powers in the kilowattrange to emit the excitation signal. At least one amplifier is thereforeprovided that feeds a signal to the input of the radio-frequencyantenna. Instead of a single amplifier for the entire antenna, it hasbeen proposed to use multiple amplifiers with output signals thatexhibit adjustable phase differences relative to one another, in orderto respectively feed separate antennas. The intensity distribution ofthe transmission field within the excitation volume should thus bebetter adapted to the requirement of the magnetic resonance. Ideally,instead of one amplifier that delivers the total power, eight individualamplifiers are then required from which only one eighth of the totaltransmission power must be delivered.

So that the amplifier can operate ideally, the connected load impedancemust always correspond to 50 Ohm. Depending on the weight and volume ofthe patient to be examined, however, the antenna impedance changes, andthus the load impedance for the amplifier also changes.

However, a pre-adaptation can only be implemented for a standard loadcase, such that in most cases a portion of the power sent to the antennais reflected at the feed point and arrives back at the amplifier. Inorder to solve this problem, the following possibilities are known.

First, the reflected power can simply be allowed to transduce into heatin the amplifier, which leads to an over-dimensioning of the coolingrequirement and the structural size. This is reflected in high costs ofthe amplifier. Additionally, the risk exists that the current andvoltage peaks of the reflection power can destroy the end stage of theamplifier.

Another possibility, in which the amplifier is protected from reflectedpower, is to use what is known as an adaptation tuner that minimizes thereflected power for every load case before the actual measurement. Inparticular given use of multiple amplifiers, such a procedure hasdisadvantages. A much greater circuit complexity is required for theadaptation tuner and the time cost for the load compensation is greater.

Therefore it has been proposed to interpose a circulator between theamplifier and the terminal of the magnetic resonance antenna, whichcirculator relays the transmission power arriving from the amplifiernearly without loss to the magnetic resonance antenna while thereflected power arrives at the third output of the circulator and thereis consumed in a power dump (thus a reflection-free power termination)or is converted into heat.

A circulator is a non-reciprocal radio-frequency component. In thepresent case a circulator with three terminals is used. An idealcirculator ensures that a signal is relayed from one terminal to anotherin one direction practically without attenuation and free of reflection.For example, a signal can be relayed only from terminal 1 to terminal 2,from terminal 2 to terminal 3 or from terminal 3 to terminal 1. In orderto achieve this non-reciprocal transmission, microwave ferrites are usedthat are saturated by a strong magnetic field. The ferrite structureused (also called a resonator) is essentially formed by two generallycylindrical ferrite plates between which a conductor trace structure isenclosed. The required constant magnetic field is generated by permanentmagnets arranged above and below the ferrite structure. A housing orcover often serves as a yoke to close the magnetic circuit. Magneticfield lines are in principle closed. To generate a predictable magneticfield it is typical to merge the magnetic field into a magnetic circuitthrough components that are particularly good conductors in order tooptimally avoid scatter field losses. Via such field conductor elements,the magnetic flux can be directed in specific, desired paths.

In order to obtain an optimally ideal circulator, it is important tofind the correct operating point, thus in particular to find an optimaloperating field. For real circulators it is additionally known that atemperature dependency exists. Known circulators accordingly require astatic magnetic operating field of specific size which is produced bypermanent magnets.

In magnetic resonance apparatuses or magnetic resonance antenna devices,cable damping (i.e. losses due to cables that are too long) should beoptimally minimized, such that the amplifier should be arranged inproximity to the antenna. The power demand at the amplifier is therebydistinctly lowered. For the possibility of a decoupling of the reflectedpower via circulators, this means that these must likewise be arrangedin the region of the antenna, but when the circulators are placed atthat location, the strong magnetic scatter field of the magneticresonance apparatus alters the field that is present in the region ofthe ferrite structure away from the optimal operating field such thatthe circulator loses its function. The magnetic properties of thepermanent magnets additionally suffer a lasting change due to the strongscatter field of the magnetic resonance apparatus. Placement inproximity to the magnetic resonance apparatus is not reasonable in thiscase.

To solve this problem it has been proposed to use a circulator withoutpermanent magnets that utilizes the fundamental “interfering” scatterfield of the magnetic resonance apparatus. For this, the circulatorwould have to be mounted at suitable positions in the scatter field atwhich orientation and size of the magnetic field coincide with theoptimal operating field as [[it]] was generated by the previouslypresent, but now removed, permanent magnets. However, ultimately thissolution would be practical only in rare cases since design limitations(for example the attachment, the cooling or the wiring) to arrange thecirculator exactly at the matching points in the scatter field are notallowed. An additional problem is that the scatter field is not constantover time, and disruptions thus can also occur.

SUMMARY OF THE INVENTION

An object of the invention is to provide a circulator that can be usedunder a larger number of external scatter field conditions and thereforecan be freely placed.

This object is achieved in a circulator of the aforementioned typewherein, according to the invention, a field conductor device fashionedto amplify or attenuate a specific external magnetic field and/or atleast one electromagnet to generate an operating field runningperpendicularly through the ferrite structure is provided instead of apermanent magnet.

According to the invention, not only is the permanent magnet omitted,but it is replaced with a field-affecting field conductor device and/oran electromagnet that can be controlled to generate a specificelectromagnetic field. By these measures it is possible to correctdeviations of the scatter field from the optimal operating field, and inextreme cases even to counteract the scatter field to generate theoptimal operating field. Within the scope of the invention it isaccordingly possibly not only to use the external magnetic field and tocorrespondingly shape it by means of the field conductor device and/orthe at least one electromagnet, but also to completely reshape it givencorresponding design of the components, such that a more functionalcirculator is ensured. For example, a field conductor device whosecomponents or field conductor elements exhibit a high magneticpermeability can be used in order to amplify an external magnetic fieldthat is too weak at the location of the ferrite structure, as well as toshield against and therefore attenuate an external magnetic field thatis too strong. Direction corrections are also conceivable givencorresponding design of the field conductor device. A controllableelectromagnetic field can be generated by the electromagnet that issuperimposed on the external magnetic field (in particular the scatterfield) in the region of the ferrite structure and supports, attenuates,but also (under the circumstances) replaces or overcompensates, theexisting external magnetic field. For example, in a combination of bothmeasures the field conductor device can enable a rough adaptation and afine correction can ensue with the electromagnet.

In this way it is possible to practically use circulators between theamplifiers and the magnetic resonance antenna so that the amplifiers canbe designed for lower maximum voltages or currents. This leads to areduction of costs, structural size and cooling capacity. By the use ofthe field conductor device and/or the electromagnet, the circulator canadvantageously be arranged at nearly arbitrary points within the scatterfield of a magnetic resonance apparatus, in particular even mounteddirectly on the magnet housing. Mechanical and design limit conditionsthat enable an actual use in the first place can thereby be satisfied.

Since a homogeneous magnetic field is required for operation of thecirculator in the region of the ferrite structure, homogenizationelements (in particular iron plates) for the homogenization andperpendicular alignment of the operating field in the region of theferrite structure can be appropriately provided on the top and bottom ofthe ferrite structure. The homogenization elements (frequently alsocalled pole plates) advantageously have at least the dimensions of the(in particular cylindrical) ferrite structure and consequently coverthis on both sides. Since the field lines emanate perpendicularly fromthe material of the homogenization element, a homogeneous operatingfield that is in particular aligned perpendicular to the ferrite platesis ensured.

As mentioned, the field conductor device can serve to shield against theexternal magnetic field. For this purpose, the field conductor devicecan be provided with a shield housing surrounding the ferrite structureand conducting the magnetic field around said ferrite structure. Thefield conductor device (whose components a composed essentially of amaterial with a high magnetic permeability) in this case acts as a typeof “bypass” since, due to the high value for the magnetic permeabilityof the shield housing, the field lines preferably run in the shieldhousing, so a reduced magnetic field is present inside the housing.

If an electromagnet is provided in addition to the field conductordevice, the shield housing can at least partially form a portion of themagnetic circuit to close the field lines of the magnetic fieldgenerated by the electromagnet. The shield housing then acts in themanner of a yoke since the field lines of the magnetic field generatedby the electromagnet also preferably run through the shield housing. Thescatter field losses of the electromagnetic field are reduced in thisway. A field conductor element forming a portion of the magnetic circuitcan appropriately be provided on the side of the ferrite structuresituated opposite an electromagnet and/or between the electromagnet andthe shield housing. Air gaps within the housing are also closed in thisway so that in particular a complete, closed circuit results. However,in the design of such an embodiment care is to be taken that a portionof the external magnetic field is conducted through the housing via thefield conductor elements and the electromagnet (as well as its core, ifpresent) in this embodiment. However, this effect can even be desired.

If, in the case of a shield housing, the shielding effect should not beimpaired, or if no shield housing at all is provided, a yoke to closethe field lines of the field generated by the electromagnet can beprovided that, for example, acts directly at the electromagnet and onthe other side at the ferrite structure or a homogenization element.

As mentioned, however, the field conductor device can also serve toamplify an external magnetic field. In an advantageous embodiment of theinvention, the field conductor device can be provided with at least one(in particular at partially conically shaped) field conductor element,arranged coaxial to the ferrite structure, to amplify or attenuate theexternal magnetic field in the region of the ferrite structure. Due tothe high permeability of the field conductor element, the field linesadvantageously run within said field conductor element. If this shapenow tapers toward the ferrite structure, the field lines are bundled andthe field is thus amplified in the tapered part. However, it is just aseasily possible that this embodiment serves to attenuate the externalfield if the cross section of the field conductor element expands as itproceeds toward the ferrite structure. Arbitrary operating fields can begenerated for a fixed, external magnetic field via the specific shaping.

As mentioned, with the electromagnet it is possible to generate acontrollable electromagnetic field so that the circulator is notoptimized for a specific position (i.e. a specific external magneticfield) given use of an electromagnet; rather, a multifaceted usagecapability exists. However, adaptation possibilities to differentexternal magnetic fields, even to other types, are conceivable.

The magnetic circuit that closes the field lines of the field generatedby the electromagnet or the field conductor path of the field lines ofthe external field in a circulator can have an air gap whose width isadjustable. As already mentioned above, an air gap leads to scatterfield losses so that ultimately the strength of the operating field atthe ferrite structure is adjustable via the width of the air gap. Theadjustment can ensue via a corresponding, externally controlledadjustment device (driven via a motor, for example, but simpleembodiments are also conceivable in which a different number of ironlamina or lamina made of other ferromagnetic materials can be introducedinto the air gap.

In an additional variant for adaptation to different positions ordifferent external magnetic fields or conditions, the circulator has adisplacement device to tilt the circulator in the mounted state or adisplacement device is associated with the mounted circulator. Changesto the field direction of the external magnetic field can be canceledout in this way. The circulator can be brought into various positionsthat can be determined according to the external magnetic field.

An external magnetic field (for example the scatter field of a magneticresonance apparatus) is frequently subjected to fluctuations that canalso have an influence on the functionality of the circulator. In aparticularly advantageous embodiment of the invention it can thereforebe provided that the circulator comprises a magnetic field sensor,wherein the feed of current to the electromagnet and/or the width of theair gap and/or the tilt position of the circulator can be adjusted basedon the measurement values of the magnetic field sensor. For thispurpose, a control device can be provided that detects the measurementvalues of the magnetic field sensor (which can fashioned as a Hallprobe, for example) and, in the given embodiment of the circulator,determines in which settings an optimal operating field is achieved bythe ferrite structure. Such a control device can then correspondinglycontrol the different components (the electromagnet and/or theadjustment device and/or the positioning device).

Not only does a variation of the external magnetic field have an effecton the capacity of the circulator, but also the optimal operating fielddepends on the temperature in the region of the ferrite structure. In afurther embodiment, the circulator therefore has a temperature sensor,and the feed of current to the electromagnet and/or the width of the airgap can be adjusted based on the measurement values. For example, such atemperature sensor can be arranged as a pad directly at the ferritestructure. Temperature changes arise not only through the environmentbut also, for example, via the operation of the electromagnet. Here aswell a control device can be provided that reads out the temperaturesensor and in which a mathematical relation or value table is storedthat sets the measured temperature in relation to an optimal operatingfield. The adjustment of the devices affective the magnitude of theoperating field can then correspondingly ensue.

It is particularly appropriate when a magnetic field sensor and atemperature sensor are provided with which a common control device isassociated that activates the corresponding devices of the circulator.As optimal an operating field as is possible is then always ensuredunder the various conditions.

As mentioned, the operation of the electromagnet (that should bedesigned to be sufficiently strong) generates heat. Therefore thecirculator can be provided with a cooling device for liquid coolingand/or radiant cooling and/or as a heat exchanger. A cooling element ofthe cooling device can be arranged, for example, on the side of theferrite structure opposite the electromagnet, for example a coolingblock through which water flows. If this is additionally designed as afield conductor element, it fulfills a dual function: as a coolingelement and as a field conductor element of a magnetic circuit or in thefield conduction path of the field lines of the external magnetic field.

It is not necessary to be limited only to one electromagnet. Forexample, two (in particular identical) electromagnets can be provided onopposite sides of the ferrite structure, analogous to the knownembodiment with one permanent magnet on each side of the ferritestructure given a conventional circulator. Such a symmetricalarrangement simplifies the adjustment of the optimal operating field, inparticular with regard to its homogeneity as well.

For example, the electromagnet can have a coil in the range from 80-120mH. Fields in the range from multiple hundreds of mT still exist even inthe border regions in a scatter field (for example of a magneticresonance apparatus), for example. If a suitably high current isapplied, a scatter field at this level can be sufficiently attenuated oreven compensated or over-compensated with such a coil.

In order to further strengthen the effect of the electromagnet, theelectromagnet can have a core. In particular, such a core can beextended over the length of the coil in order to contact other fieldconductor elements, for example.

In addition to the circulator, the invention also concerns a magneticresonance antenna device for a magnetic resonance apparatus thatincludes a magnetic resonance antenna with at least one terminal and atleast one amplifier associated with a terminal to activate the magneticresonance antenna. The magnetic resonance antenna device has acirculator according to the invention that is interconnected between theamplifier and the terminal so that power reflected from the magneticresonance antenna can be discharged into a power dump connected with thecirculator. All embodiments of the circulator according to the inventionthat have been described in the preceding can be realized in such amagnetic resonance antenna device. In particular, the magnetic resonanceantenna device can have a displacement device coupled in terms ofmovement with the circulator to tilt said circulator. In this case thedisplacement device does not form part of the circulator but rather isassociated with it. If multiple amplifiers for different antennaelements are provided, a circulator can naturally be associated witheach of these amplifiers and antenna elements.

The invention also concerns a magnetic resonance apparatus that has amagnetic resonance antenna device according to the present invention.Via the use of the circulator according to the invention in a magneticresonance apparatus, a greater freedom is provided in the arrangement ofthe circulator, which provides corresponding freedom as to themechanical and design limit conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a magnetic resonance antenna deviceaccording to the invention.

FIG. 2 shows a first exemplary embodiment of a circulator according tothe invention in cross section.

FIG. 3 shows a second exemplary embodiment of a circulator according tothe invention, in cross-section, wherein the associated activation isonly schematically represented,

FIG. 4 shows a third exemplary embodiment of a circulator according tothe invention, in cross-section.

FIG. 5 shows a magnetic resonance apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the circuit diagram of a magnetic resonance antenna device1 according to the invention. It has an amplifier 2 to amplify thetransmission signals to be emitted by a magnetic resonance antenna 3. Acirculator 4 according to the invention and that has three terminals I,II and III is connected between the amplifier 2 and the magneticresonance antenna 3. If the correct operating field is present at theferrite structure of the circulator 4 (not shown in detail here),signals are relayed from the terminal I to the terminal II, from theterminal II to the terminal III and from the terminal III to theterminal I, as indicated by the arrow. In this way power reflected fromthe magnetic resonance antenna 3 is essentially entirely fed via theterminal III to a power dump 5, thus a reflection-free terminator. Theamplifier 2 therefore does not need to be over-dimensioned.

The circulator 4 should be suitable to be operated in the externalmagnetic field of a magnetic resonance apparatus, so an optimally wideselection of positioning possibilities should be available. Variousembodiments of the circulator 4 are presented with reference to FIGS. 2through 4.

FIG. 2 shows a first embodiment of a circulator 4 a in cross section. Itcomprises a ferrite structure 6 that is covered on both sides by ironplates 7 that serve as homogenization elements and generate between thema homogeneous magnetic field aligned perpendicular to the ferritestructure 6.

Due to the cross section view, only the terminals I and II are visible.The circulator 4 a is arranged in an external magnetic field that inthis case is too weak to serve as an operating field for the ferritestructure 6. Therefore a field conductor device is provided that has afield conductor element 8 tapering conically toward the ferritestructure 6. This causes the field lines to be compressed toward theferrite structure 6 (as shown by the arrows representing the fieldcurve) so that a stronger field exists there that can serve as anoperating field for the ferrite structure 6. The field is relayedoutward again by means of an additional field conductor element 9 thatis arranged on the side opposite the field conductor element 8. Anon-magnetic housing 10 is provided to stabilize the circulator 4 a.With a simple design it is accordingly possible to shape the externalmagnetic field so that it can be used as an operating field for theferrite structure 6. Moreover, it is noted that the circulator 4 a canalso be slightly angled relative to the external field since—due to thelimit conditions—the field lines always enter into the field conductorelement 8 perpendicularly, such that the angle is corrected again. Givenvarying orientations of the external field, a displacement device canalso be provided by means of which the circulator 4 a can be pivoted.This is not shown in detail here.

The field conductor elements 8 and 9 are composed of a material withoptimally high magnetic permeability in order to conduct the field asoptimally as possible.

A second embodiment of the present invention is shown in FIG. 3.Components of the circulator 4 b that coincide with those of the firstexemplary embodiment are designated with the same reference characters.A ferrite structure 6 is again provided that, in this case however, isnot enclosed by iron plates, although this would also be possible here.The circulator 4 b comprises an electromagnet 11 that is geometricallydimensioned so that it can be placed precisely on the ferrite structure6 with its central passage opening. An optimally direct connection ofthe ferrite structure 6 to a core 12 of the electromagnet 11 can berealized in this way. The core 12 is extended so that it is in contactwith the shield housing 13 that completely surrounds the arrangement.The shield housing 13 is part of the field conductor device and also itserves as a yoke to close the field lines of the field generated by theelectromagnet 11. When completely closed, this magnetic circuit is by acooling element 14 arranged on the opposite side of the ferritestructure 6 in relation to the electromagnet 11, which cooling element14 is designed so that it also serves as a field conductor element.

As mentioned, the shield housing 13 is part of the field conductordevice. A good portion of the field lines of the external field areconducted by this around the inside of the shield housing 13, as isagain indicated by the arrows. Another portion of the field lines of theexternal field traverse the inside of the shield housing 13 across thecore 12, the ferrite structure 6 and the cooling element 14. The fieldconductor device (comprising the shield housing 13, the core 12 and thecooling element 14) is now designed so that the external magnetic fieldis attenuated such that it can essentially serve as an operating fieldfor the ferrite structure 6. In order to generate an optimal operatingfield, the electromagnet 11 is correspondingly fed with current.

The cooling element 14 belongs to a cooling device 15 fashioned as awater cooler that is provided in order to counteract the heatdevelopment due to the electromagnet 11. Water supplied via the feed anddischarge line 16 is conducted through channels provided inside thecooling element 14. The activation of the cooling device 15 ensues bymeans of a control device 17 via which the electromagnet 11 and adisplacement device (indicated at 18 only for clarity) with whose helpthe circulator 4 b can be tilted are also activated.

According to everything stated in the preceding, given a fixed currentfeed to the electromagnet 11 the magnetic field present in the ferritestructure 6 is dependent on the strength and possibly orientation of theexternal magnetic field. However, at the same time it applies that theoptimal operating field is dependent on the temperature of the ferritestructure 6. However, the circulator 4 b is fashioned to reactdynamically to such variations of the operating conditions and thus toachieve an optimal operating field (and thus an optimal functionality)of the circulator at any time. For this purpose, the circulator 4 bcomprises a magnetic field sensor 19 arranged in the shield housing 13in the form of a Hall probe with which the external magnetic field canbe measured. Furthermore, a temperature sensor 20 is provided betweenthe core 12 and the ferrite structure 6, thus directly adjoining theferrite structure 6. Both the magnetic field sensor 19 and thetemperature sensor 20 deliver their measurement data to the controldevice 17 where, using the data about the external magnetic field, it ischecked whether the optimal operating field required based on thetemperature measurement data is still maintained given the currentoperating conditions. If that is not the case, the control device isfashioned to activate the electromagnet 11 and the displacement device18 so that an optimal operating field is again present in the ferritestructure 6.

Not shown in detail in FIG. 3 is an additional possibility to adjust themagnetic field in the ferrite structure 6 that can also be used in thefirst exemplary embodiment. An adjustable air gap can thus be providedin the magnetic circuit or, respectively, on the path of the field linesof the external field. For example, it would be conceivable to detachthe upper part of the core 12 and to direct it through the shieldhousing 13 so that its distance from the lower part of the core 12 canbe varied. An adjustment device that can also be controlled by thecontrol device 17 can be provided for this. An additional degree offreedom for the adjustment of the field results in this way sincescatter field losses occur due to the air gap. This means that thegreater the air gap, the smaller the field at the ferrite structure 6.

The control device 17 does not have to be arranged outside of thecirculator 4 b, as shown here; rather, it can naturally also beadvantageously integrated into this. The positions of the sensors canalso be selected differently.

FIG. 4 shows a third exemplary embodiment of a circulator 4 c accordingto the invention, wherein here the activation electronics and themagnetic field are not shown for clarity. A ferrite structure 6 isarranged between two iron plates 7 as homogenization elements.Respectively arranged on opposite sides of the ferrite structure 6 is anelectromagnet 11 a, 11 b, wherein the electromagnets 11 a and 11 b arecoils of identical design. The magnetic circuit is closed by a yoke 21.A temperature sensor 20 and a magnetic field sensor 19 are also providedin turn in this case.

Naturally, a shield housing can also additionally be provided at thecirculator 4 c so that ultimately less field penetrates inside and theoperating field is essentially determined by the electromagnets 11 a and11 b. Other field conductor elements can also be used. Additionally, itis also reasonable here to provide a cooling device 15 (which is onlyindicated in FIG. 4, however).

Additional adjustment possibilities can also be realized in thecirculator 4 c. For example, the yoke 21 can be adjusted in terms of itsheight so that an air gap can be formed in the magnetic circuit. Adisplacement device to tilt the circulator 4 c can also be associatedwith said circulator 4 c, or this can possess such a displacementdevice.

The electromagnets 11, 11 a and 11 b each can be fashioned as coils. Ifthe circulator 4 is provided for use in a magnetic resonance apparatus,for the most part fields in the range of multiple tens or multiplehundreds of mT are used. For comparison: the optimal operating field ofa ferrite structure 6 can be approximately 47 mT at 30° C., for example,approximately 40 mT at 70° C. For an operation under such conditions thecoils can exhibit inductances of 80 to 120 mH, for example.

FIG. 5 shows a magnetic resonance apparatus 22 according to theinvention that has a magnetic resonance antenna device 1 according tothe present invention.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted heron all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

1.-19. (canceled)
 20. A circulator for use in an external magnetic fieldof a magnetic resonance apparatus, comprising: a flat ferrite structure;and an operating field generator located relative to said flat ferritestructure that generates an operating field proceeding perpendicularlythrough said ferrite structure, said operating field generator beingselected from the group consisting of a field conductor device thatamplifies or attenuates said external magnetic field, and at least oneelectromagnet.
 21. A circulator as claimed in claim 20 comprisinghomogenization elements located at a top and bottom of said ferritestructure that homogenize and perpendicularly align said operating fieldin said ferrite structure.
 22. A circulator as claimed in claim 20comprising a shield housing that surrounds said ferrite structure andconducts said external magnetic field around said ferrite structure. 23.A circulator as claimed in claim 22 wherein said operating fieldgenerator is an electromagnet, and wherein said shield housing forms atleast a portion of a magnetic circuit that closes field lines of amagnetic field generated by said electromagnet.
 24. A circulator asclaimed in claim 23 comprising a field conductor element, forming aportion of said magnetic circuit, located at a side of said ferritestructure opposite said electromagnet, or located between saidelectromagnet and said shield housing.
 25. A circulator as claimed inclaim 20 wherein said operating field generator is an electromagnet, andcomprising a yolk that closes field lines of a magnetic field generatedby said electromagnet.
 26. A circulator as claimed in claim 20 whereinsaid operating field generator is a field conductor device, and whereinsaid field conductor device comprises at least one field conductorelement located coaxially relative to said ferrite structure thatamplifies or attenuates said external magnetic field in said ferritestructure, said field conductor element being at least partiallyconically shaped.
 27. A circulator as claimed in claim 20 comprising amagnetic circuit that closes field lines of a magnetic field generatedby said operating field generator within an air gap, and wherein a widthof said air gap is adjustable.
 28. A circulator as claimed in claim 27comprising a magnetic field sensor that measures said external magneticfield and generates a sensor output signal corresponding thereto, andwherein said magnetic circuit is supplied with said sensor output signaland adjusts the width of said air gap dependent on said sensor outputsignal.
 29. A circulator as claimed in claim 27 comprising a temperaturesensor that senses a temperature of an environment in which said ferritestructure is located, and generates a sensor output signal correspondingthereto, said magnetic circuit being supplied with said sensor outputsignal and adjusting the width of said air gap dependent thereon.
 30. Acirculator as claimed in claim 20 comprising a displacement device thattilts said ferrite structure relative to said external magnetic field.31. A circulator as claimed in claim 30 comprising a magnetic fieldsensor that measures said external magnetic field and generates a sensoroutput signal corresponding thereto, said sensor output signal beingsupplied to said displacement device and said displacement devicecontrolling tilting of said ferrite structure dependent on said sensoroutput signal.
 32. A circulator as claimed in claim 30 comprising atemperature sensor that senses a temperature of an environment in whichsaid ferrite structure is located, and that generates a sensor outputsignal corresponding thereto, said sensor output signal being suppliedto said displacement device and said displacement device controllingtilting of said ferrite structure dependent thereon.
 33. A circulator asclaimed in claim 20 wherein said operating field generator is anelectromagnet, and wherein said electromagnet operates with a currentflowing therein, and comprising a magnetic field sensor that measuressaid external magnetic field and generates a sensor output signalcorresponding thereto, said sensor output signal being supplied to saidelectromagnet and said electromagnet controlling said current dependentthereon.
 34. A circulator as claimed in claim 33 wherein said operatingfield generator is an electromagnet having a current flowing therein,and comprising a temperature sensor that senses a temperature in anenvironment in which said ferrite structure is located, and thatgenerates a sensor output signal corresponding thereto, said sensoroutput signal being supplied to said electromagnet and saidelectromagnet controlling said current dependent thereon.
 35. Acirculator as claimed in claim 20 comprising a cooling device that coolssaid ferrite structure, said cooling device being selected from thegroup consisting of liquid cooling devices, radiant cooling devices, andheat exchangers.
 36. A circulator as claimed in claim 35 wherein saidoperating field generator is an electromagnet, and wherein saidelectromagnet is located at a side of said ferrite structure, andwherein said cooling device comprises a cooling element located at aside of said ferrite structure opposite to the side of said ferritestructure at which said electromagnet is located.
 37. A circulator asclaimed in claim 20 wherein said operating field generator comprises twoidentical electromagnets respectively located on opposite sides of saidferrite structure.
 38. A circulator as claimed in claim 20 wherein saidoperating field generator is an electromagnet, and wherein saidelectromagnet comprises a coil having an inductance in a range between80 and 120 mH.
 39. A circulator as claimed in claim 20 wherein saidoperating field generator is an electromagnet, and wherein saidelectromagnet comprises a core.
 40. A magnetic resonance antenna devicefor a magnetic resonance apparatus comprising: an antenna configured totransmit radio-frequency signals or receive magnetic resonance signals,said antenna comprising at least one antenna terminal; an amplifierconnected to said at least one antenna terminal configured to activatesaid antenna; a circulator connected between said amplifier and said atleast one antenna terminal that discharges power reflected by saidantenna into a power dump connected to the circulator; and saidcirculator comprising a flat ferrite structure, and an operating fieldgenerator located relative to said flat ferrite structure that generatesan operating field proceeding perpendicularly through said ferritestructure, said operating field generator being selected from the groupconsisting of a field conductor device that amplifies or attenuates saidexternal magnetic field, and at least one electromagnet.
 41. A magneticresonance antenna device as claimed in claim 40 comprising adisplacement device that mechanically tilts said circulator relative tosaid external magnetic field.
 42. A magnetic resonance apparatuscomprising: a magnetic resonance data acquisition device comprising anantenna that radiates radio-frequency signals or receives magneticresonance signals, said antenna having at least one antenna terminal; anamplifier connected to said at least one antenna terminal configured toactivate said antenna; a circulator connected between said amplifier andsaid at least one antenna terminal that discharges power reflected bysaid antenna into a power dump connected to the circulator; and saidcirculator comprising a flat ferrite structure, and an operating fieldgenerator located relative to said flat ferrite structure that generatesan operating field proceeding perpendicularly through said ferritestructure, said operating field generator being selected from the groupconsisting of a field conductor device that amplifies or attenuates saidexternal magnetic field, and at least one electromagnet.